1. Field of the Invention
The present invention relates to a metallic pipe having a relatively small diameter of about 20 mm or less and arranged as a piping such as the brake pipe or fuel pipe of an automobile in a lower portion of the body. More particularly, the invention relates to both a metallic pipe having on its outer circumference multiple sheaths having resistances to the chipping (of pebbles) and splashing (of muddy water) during a running of the automobile and excellent corrosion resistances, and a method of sheathing the metallic pipe.
Here in the specification, the term "stepped" generally means generally upright, sloped or curved faces at the two longitudinal sides of the metallic pipe.
2. Description of the Prior Art
As the coated metallic pipe of this kind, there have been known in the prior art: a metallic pipe which is prepared by forming a chromate film, if necessary, on a Zn or Zn/Ni plated film applied to the outer circumference of the metallic pipe, by coating the chromate film with polyvinyl fluoride (PVF) having a thickness of about 20 microns, and by covering the coating with a heat-shrinkable tube; and a metallic pipe which is prepared by extruding a polyamide resin (PA) such as PA11 or PA12 having a thickness of about 200 microns on a Zn or Zn/Al plated film applied to the outer circumference of the metallic pipe.
In the former prior art. however, the expensive heat-shrinkable tube is used to raise the production cost, and the step of heating to shrink the heat-shrinkable tube applied to the polyvinyl fluoride layer is required to trouble the manufacture and deteriorate the productivity. In the latter prior art. on the other hand, the use of the single-layered thick film of a polyamide resin causes the defects such as the pin holes to continue in the thickness direction thereby to raise a problem that the corrosion resistance is deteriorated.
When the coated metallic pipe of the two prior arts is to be arranged in an automobile, moreover, the overcoat layer has to be scraped off when the metallic pipe is worked at its two ends for joints. This necessity raises a problem that the cutting edge for the scraping operations damages the outer circumference or the protective film of the metallic pipe thereby to deteriorate the corrosion resistance.
When the coated metallic pipe of the latter prior art is to be arranged in an automobile, it is mounted to its mating member through an insulator. Because of the large thickness of the coated layer, the insulator has to be especially large-sized. and the metallic pipe has a large diameter to raise problems that the bending radius is enlarged and that the piping cannot be made in a predetermined space.